Composite flow meter
BAKCGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a composite flow meter, and more particularly, to a flow meter of this kind which is composed of a Karman vortex flow meter and a mircobridge flow sensor. 
     2. Description of the Prior Art 
     A composite flow meter conventionally employed in the art is such one that is composed of a microbridge sensor 1 and a fluidics flow meter 2 as shown in FIG. 1. 
     Such conventional composite flow meter composed of the microbridge sensor 1 and the fluidics flow meter 2 has a problem in that the complicated structure and large size of the fluidics flow makes it difficult to mount the microbridge sensor 1 at a location in the fluidics flow meter 2 and also reduce the size of the whole flow meter. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     In view of the above-mentioned problem, it is the object of the present invention to provide a composite flow meter which is in a simple and small structure and can be manufactured at a low cost. 
     To achieve the above object, there is provided a composite flow meter comprising: 
     a Karman vortex flow meter for measuring a high flow range; and 
     a thermal flow sensor for measuring a low flow range. 
     Preferably, the output signal value derived from the thermal flow sensor is corrected by the output signal value derived from the Karman vortex flow meter. 
     The above and other objects and features of the present invention will become apparent from the following detailed description of the preferred embodiments with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a conventional composite flow meter; 
     FIG. 2 is a cross-sectional view of an embodiment of the present invention; 
     FIG. 3 is a side view taken from the direction shown by an arrow A; and 
     FIG. 4 is a graph showing output signal levels of the respective flow meter and sensor. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2 shows an embodiment of a composite flow meter according to the present invention. The shown composite flow meter includes a microbridge sensor 1, a Karman vortex flow meter 2a, a screen 3 for preventing disturbance in flow, a vortex generator 4 for generating Karman vortex and a pressure measuring bore 5 for detecting vortex. An arrow F indicates the direction of the flow. 
     The present embodiment measures a flow in a large amount range with the Karman vortex flow meter 2a and a flow in a small amount range with the microbridge sensor 1. 
     Such combination of the microbridge sensor 1 and the Karman vortex flow meter 2a makes its structure simpler, its size smaller, the mounting of the microbridge sensor 1 easier and its manufacturing cost lower than the conventional composite flow meter formed of a fluidics flow meter and a microbridge sensor. 
     FIG. 3 is a side view of the composite flow meter taken from the direction shown by an arrow A. In the same drawing, reference numeral 6 designates a tube and 7 a pressure/electric converting sensor. 
     Next, a method of correcting the output signal value from the microbridge sensor 1 with the output signal value from the Karman vortex flow meter 2a with reference to FIG. 4. 
     In FIG. 4, a curve S FA  is a characteristic curve indicative of the output signal level from the microbridge sensor 1, a curve S FB  a characteristic curve indicative of the output signal level from the Karman vortex flow meter 2a, and a curve S FAC  a characteristic curve indicative of the corrected output signal level from the microbridge sensor 1. The abscissa of the graph represents the flow (F) and the ordinate the output signal level. The correction is effected between output signal levels F1 and F2. If it is assumed, for example, that the output level of the microbridge sensor 1 is Fb and that of the Karman vortex flow meter 2a Fa when the flow is FA, the correction coefficient k is calculated by dividing Fa by Fb (Fa/Fb). Therefore, the corrected output signal level Fb&#39; of the microbridge sensor 1 is calculated by the following equation; 
     
         Fb&#39;=k·Fb=Fa 
    
     By this correction applied to the output of sensor 1 by a suitable circuit 8, continuity of the output signal can be maintained even if the microbridge sensor 1 is changed over to the Karman vortex flow meter 2a, and vice versa. 
     The output signal levels F1 and F2 for determining a correction range of the output signal are selected in the following manner. Assuming that a lower limit value which can be correctly measured by the Karman vortex flow meter 2a is represented by FO, and an upper limit value which can be correctly measured by the microbridge sensor 1 by F3, the levels F1 and F2 are selected so as to satisfy the following equation: 
     
         FO≦F1&lt;F2≦F3 
    
     The change-over between the microbridge sensor 1 and the Karman vortex flow meter 2a is effected between the output signal levels F1 and F2. The change-over can be automatically carried out by a microcomputer (not shown) which is provided with the output signal level from the Karman vortex flow meter 2a. Also, a signal processing circuit including a comparator may be used in place of the microcomputer. 
     Incidentally, since the Karman vortex flow meter is generally adapted to measure a narrower flow range than the fluidics flow meter, it is necessary to extend the measuring range of the microbridge sensor so that the composite flow meter of the present invention is applicable to the same flow range as the fluidics flow meter. 
     According to the present invention as described above, the composite flow meter can be made simple and small at a low cost by employing a Karman vortex flow meter for measuring a high flow rate range and a microbridge sensor for measuring a low flow rate range. 
     Further, the output signal value derived from the microbridge sensor is corrected by the output signal value from the Karman vortex flow meter which is more accurate than the microbridge sensor (thermal flow meter) for a long period, to thereby maintain the continuity and accuracy of the output signal in the whole range.